This invention relates to a navigation system, a method of navigation, and a navigation computer program especially for an automobile. The navigation system for an automobile which has a display for displaying a moving map in accordance with a estimated present position and road database is known in U.S. Pat. No. 4,535,335 and U.S. Pat. No. 4,468,581. Further map matching technology in which the estimated present position is corrected based upon a road database to correct errors in the estimated present position data which can include an integral error caused by sensor error is also known in U.S. Pat. No. 4,660,161. U.S. Pat. No. 5,323,321 discloses navigation system which includes maneuver guidance which uses a maneuver graphic arrow while the vehicle moves along guidance route. The guidance route is determined based on starting position, e.g. present position before driving, and destination which is inputted by the driver before the guidance route is calculated.
However, if the driver misses making a turn in accordance with the maneuver direction at a corner, or if the driver intentionally passes a corner in order to make a turn at another corner, the navigation system indicates that the vehicle is off-route. After off-route detection, a recalculating method for a new optimum route for a destination is known in U.S. Pat. No. 5,291,414. Based on the prior art for determining the new route when the driver misses the turn at the maneuvered corner, the CPU calculates from the starting position to calculate toward the destination. In the prior art, the vehicle continues to move while the CPU is calculating a new route, and the driver could miss the turn at a new maneuvered corner if the navigation system instructs the new maneuvered corner after passing or just before the corner. The prior art system thus determines a starting position in place of the present position for recalculation to take the new optimum route by shifting forward a certain distance from the present position. U.S. Pat. No. 5,291,414 discloses that distance between present position and the starting point for making recalculation should change in accordance with vehicle speed. However, this technology could still cause an off-route condition if the destination is a far place. In this case since a long period of time is necessary to finish recalculation and the vehicle is driven while the recalculation is executed, a driver could miss a turn if the vehicle position is at or beyond the intersection at which it should make the turn based on the result of recalculation.
In order to overcome the problem in prior art, assignee's copending application, Ser. No. 08/733,745, filed Oct. 16, 1996, based upon Japanese Patent Application No. Hei 7-342646, which is incorporated herein by reference, is directed to shortening the period for recalculation. This application discloses a return point which is used for executing the recalculation to determine a new route from present vehicle position to the return point when the vehicle falls into an off-route condition. Based upon the disclosure of this application, the distance between present position and the return point, which is determined, is limited to short distance. Therefore, the chance of a second off-route condition can be reduced since period of recalculation is short.
However, if the vehicle is waiting for a traffic signal located at an intersection at which it is to take a turn towards the return point, it could be difficult to turn according to the result or recalculation which has been executed while the vehicle has stopped.